High-frequency signaling system



Nov. 20, 1928.

Y R 5 POTTER HIGH FREQUENCY SIGNALING SYSTEM E iled Dec. 2s, 192

4 Delay Circuit IN VEN TOR BY ATTORNEY Patented Nov. 20, 1928.

UNITED STATES PATENT OFFICE.

RALPH K. POTTER, OF NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

HIGH-FREQUENCY SIGNALING SYSTEM.

Application filed December 23, 1925. Serial No. 77,305. I

This invention relates to radio transmitting systems, and particularly to a modulating device characterized by means for controlling the transmission of the carrier frequency.

In high frequency transmission systems which include both radio and wire systems, it has been customary either to radiate the carrier frequency continuously or tosuppress the transmission of the carrier frequency. Sys tems in which the carrier frequency is transmitted at all times, regardless of whether modulation is or is not taking place, are not only wasteful of power but also possess other disadvantages. Systems in which the carrier frequency is at all times suppressed, that is to say, is prevented from being radiated, cannot be employed under conditions where it is necessary to supply the carrier frequency to a distant receiving station whlch has no local source of oscillations for beating the incoming band frequencies for the purpose of'de-.

tecting the signal.

This invention resides in a system of high frequency transmission in which the carrier frequency is suppressed, that is to say, its radiation is prevented except during the time in which a signaling currentis impressed upon a modulator for modulating the said carrier. I A system such as this in which the carrier is suppressed between modulation periods not only effects a saving of power but also possesses other advantages over the systems of the prior art, one of which resides in its ability to reduce the effect of-static' and power disturbances at the receiving station during the intervals between modulation periods, and another advantage resides in the ability of the system to efiect radio transmission and reception on the same wave length. The first mentioned advantage, namely, the suppression of disturbances, results from the absence of the carrier during the quiescent intervals between modulation, and it is twofold in its character; first, there is a psychological effectupon the listener at the receiving station resulting from a reduction in the noise level when modulation is not occurring at the transmitting station; and second, it appears probable that there is a distinct improvement in the effect upon perception of intelligible sounds.

This invention will be clearly understood from the following description-when read in connection with the attacheddrawing, of

which Figure 1 shows schematically a form of embodiment of the invention; and Fig. 2 is a modification of Fig. 1.

In Fig. 1, the transmitter 1 represents symbolically any form of transmitting station which is connected with the primary winding 2- of the transformer 3, which transformer is preferably of the step-up type. A secondary winding 4 is connected with the input circuit of the modulator M comprising the vacuumtubes 5 and 6 and associated apparatus by means of which modulation is effected. This modulating circuit is, with certain exceptions, of the well-known type disclosed in the patent to Carson No. 1,449,382, dated March 27, 1923, but it differs therefrom in that the carrier frequency is suppressed only when no modulation is occurring, and is transmitted together with the side bands resulting from the modulation of the carrier through the output circuit. The grids of the tubes 5 and 6 are connected with the extremities of the winding 4, which has its midpoint connected with the filaments of the tubes 5 and 6, the connection including resistances 7- and 8.-- These resistances have condensers 9 and 10 connected in parallel therewith. The common conductor of the tubes contains the winding 11 of a transformer that has its other winding 12 connected with the source of carrier oscillations 13. The condensers 14 and 15 shunt the winding 4, and permit the high frequency currents to pass directly to the grids of the tubes. The plates of the tubes 5 and 6 are connected with the extremities of the winding 16 of a transformer having its secondary winding 17 connected with an'output circuit containing a filter 18 adapted to transmit therethrough the carrier frequency and one of its side bands. The midpoint of the winding 16 is connected with the junction point of the resistances 7 and 8, the connection including the source of plate potential 19. The transformer 3 has also another secondary winding 20 which is connected with the input side of an amplifier 21, whose output side is coupled by the transformer 22 (or otherwise connected) with the rectifier tube 23. The

ot'the rectifier is bridged across the resistances 7 and 8 so as to establish across these resistances a direct current potential, the magnitude of which is proportional to variations in the speech currents impressed by winding 2 upon winding of transformer 3. The manner in which this circuit operates in order to attain the objects of the invention is as follows signaling currents arenot produced by acti- ,vation of the transmitter l, the oscillations created by the source 13 will be impressed upon the input circuits of the tubes 5 and 6 and will afiect the grids in like manner so that no variation, of the plate currents is produced, and consequently, no'trantmission of the carrier will be efiected over the output circuit which may be either a radio or a wire circuit. As-soonas'speech or other form of signal is impressed upon the transmitter 1 and a corresponding current flows through the windin 2 of the transformer 3 -corresponding voltages will be impressed across the windings 4 and 20. That'which issproduced in winding 4 afiects the grids of the tubes 5 and 6 in the well-known manner to modulate the carrier frequency from the source 13. The side bands thus produced are impressed by the winding 16 upon the winding 17, and in turn upon-the filter 18'. The voltage simultaneously created in the wind ing 20 will be amplified by 21 and rectified by the tube 23. It will be seen that the plate current fromthe source 26 flows normally through the resistances 7 and 8 one of which that two'complex currents as represented by 7 is in the grid circuit of the tube?) and the other in the grid circuit of the tube 6. The

potential difierences across said resistances when added to those of the sources 27 and 28, are normally sufliciently great to keep the grids negative to the same degree during the period in which modulation is not taking place, .and'thusa condition of balance is created which suppresses the carrier. When Here the first term represents the carrier wave, andits amplitude 1s a function of the grid. potential E The second and third terms represent the lower and upper side band components, respectively.

The circuit arrangement in Fig. 1 is such c +Ec) 2 Sin (Q 4 2) (3) are combined in opposite sense in theoutput. If the amplitude factors 6, and e are considered negative for one tube and positive for the other, the output of carrier current becomes proportional to r se. n (g -a) a. speech currentfiows through the Winding producedby 23 will tend to keep the grid of the rectifier 23 'suficiently negative during modulation- This produces an unbalancein the tubes, and consequently, 1 carrier 'osc'illavtions will no longer be balanced out but Will P be transmitted by thewinding 16 to thewind- During the period in which speech or other.

ing 17 and-wi1l be radiatedtogether with'the: side band or bands.

Qthe resultantvoltage will be amplifiedby '21 sufficiently sothat the detected voltage Within a. reasonable approximationthe plate current of a three-element vacuum tube may be enpressed by the relation wleredepends upon the structure of the tu e; p

a is the voltage amplification constant; e depends upon grid-filament contact potentialand the power developed in the filament;

E is plate potential; and

E is grid potential.

The first and last factors withinthe parenthesis of equation 1) may, for the purpose of this explanation, be assumed constant and equal to C. If thentwo waves, one of audible and one of carrier frequency, are impressed upon the input, the general relation of equation (1) becomes T=a[C+E +e sin (2 4 1) z (Q -e01 where C=E /,a l-e; Y

e =amplitude of audio wave;

e =ainplitude of carrier wave;

32/211 audible frequency;

g/21r=carrier frequency; and

t, and are the arbitrary phase angles of the audible and carrier waves.

The expansion of equation (2) and elimination of all terms except those in the vicinity of the carrier frequency gives an output cur rent proportional to 2 cos im -ares (a the input, the rectified output of tube 23 flows through resistances 7 and. 8, producing an unbalance and if normally 'E =E', the carrier current now becomes proportional to add-ma (Q -e2) v where i, is therectified output of tube 23 and r, and 1' are the resistances 7 and 8, respectively. Thus duringv modulation there is V V p produced in the output of the modulator a while the side band output containing prod-,-

'uct terms is unmodified. Here Ea is the grid carrier current proportional to (5). I

The arrangement shown in Fig. 2 illusf trates a Way in which modulation may be retarded vuntil the speech current has been rectified and effects an unbalancing of the modulating circuit, whereby modulation and effective transmission of the carrier will take occurs.

While this invention has been disclosed as embodied in a particular form, it is capable of embodiment in other anddiiferent forms without departing from the spirit and scope of the appended claims.

What is claimed is:

1, In a high frequency signaling system, the combination with a balanced modulator of a source of carrier oscillations, the said modulator being ofthe carrier suppression tv e th s aid modulator, and means controlled by the signal to unbalance the said modulator and to eflect the transmission of the carrier oscillations whenever modulation takes place.

2. In a high frequency signaling system,

a signal input circuit connected with modulator ofthe carrier suppression type, of a source of carrier oscillations connected with a conductor common to the grids of the tubes of the said balanced modulator so as to normally affect the grids to the same extent, a signaling circuit connected with the input of the said modulator, a vacuum tube rectifier having its plate circuit connected with the grid circuits of the modulator tubes so as to apply direct current potentials to the grids of.

said tubes, sources of potentialalso connected with the grids of said tubes to neutralize the bias upon the grids normally produced by the said direct current potential resulting from the plate current of the rectifier, and means to. unbalance the said modulator during modulation by the said signaling current to effect the transmission of the carrier oscillations. i

5. In a high frequency signaling system comprising a balanced modulator, a source of carrier frequency, a source of signaling frequency to modulate the said carrier fre quency, and a transmitting circuit, the

method ofsignaling which consists in preventing the transmission of the carrier frequency by maintaining the modulator in a balanced condition when modulation is not occurring, and in effecting transmission of the carrier frequency by unbalancing the modulator by the signaling current which the combination with a balanced vacuum tube modulator of the carrier suppression type of a source of carrier osclllations, a Source of modulating current, and a control circuit connected with said source of modulating current and also with said modulator comprising an amplifier and a rectifier arranged to apply unlike voltages to the grids of the modulator tubes proportional to the rectified modulating current, whereby the said modulator will be unbalanced and transmission of the carrier will occur.

3. In a" high frequenc signaling system, the combination with a alanced modulator of the carrier suppression type, of a'source of carrier oscillations connected therewith, a source of modulating current connected wit-h the said modulator, and means connected with the said source of modulating current and said modulator to unbalancethe latter and effect the transmission of carrier oscilla tions whenever modulation-takes place.

alsosimultaneously modulates the carrier frequency. 6. In a high frequency signaling system comprising a balanced modulator, a. source of carrier frequency, a source of signaling frequency to modulate the said carrier frequency, and a transmitting circuit, the method of signaling which consists in maintaining the modulator in balanced condition when modulation is not occurring, thus preventing the transmission of the carrier frequency, then modulating the carrier frequency by thesignaling frequency to produce side bands of the said carrier, and simultaneously rectifying a part of,the signaling frequency and unbalancing the modulator by the said rectified signaling frequency to cause the transmission of the carrier frequency oscillation Whenever side bands thereof are being produced.

In testimony whereof, I have signed my name to this specification this 22nd day of December, 1925. 

